time measuring circuits for measuring a phase difference between input pulses as a time have been developed, which are for example disclosed in U.S. Pat. No. 5,568,071 corresponding to Japanese Unexamined Patent Publication No. H03-220814.
The time measuring circuits of the U.S. patent publication are each composed of a plurality of digital circuits each configured to perform a particular logical function based on at least two discrete voltage levels.
Specifically, one typical example of the time measuring circuits includes a pulse delay circuit composed of a plurality of delay units that corresponds to a plurality of stages of delay. The delay units are connected to one another in series or in a ring-like structure.
In the time measuring circuit, when a starting pulse is input to one of the delay units corresponding to the first stage of delay, the starting pulse is sequentially transferred by the delay units while being delayed thereby in the order from the first stage of delay units toward the last stage thereof.
The time measuring circuit is designed to:
count a number of stages (pulse delay units) through which a pulse signal has passed since the input of the starting pulse up to an input of a measuring pulse to the time measuring circuit; and
output digital data based on the counted number of stages (pulse delay units) as a phase difference (time difference) between the starting pulse and the measuring pulse.
Such a time measuring circuit requires no analog circuits and consists entirely of a plurality of digital circuits, which makes it possible to easily design time measuring circuits as ICs (Integral Circuits).
In measurement of a time, there are requirements to measure a micro time length with a high resolution, such as a requirement for laser radars to measure a period of time elapsing between transmission of a laser beam and receipt of a reflected beam from the target. In contrast, there are requirements to measure a comparatively long time length with a comparatively low resolution, such as a requirement for ultrasonic sonars to measure a period of time elapsing between transmission of an ultrasonic wave and receipt of a reflected wave from the target.
A resolution required to measure a micro time length and that required to measure a comparatively long time length may differ from each other by ten orders of magnitude.
To meet the micro time-length measurement requirements with high resolution, in a conventional time measuring circuit, shortage of a delay time of each delay unit (each stage in delay) constituting the pulse delay circuit is needed. The shorter the delay time of each delay unit is, in other words, the higher the resolution of a conventional time measuring circuit, the more the number of stages through which a starting pulse signal has passed in the pulse delay circuit during even a predetermined same period. This may cause, in order to meet the long time-length measurement requirements with low resolution, a structure of a conventional time measuring circuit required to count the number of stages to increase in size, which may increase the conventional time measuring circuit in size.
As a different approach, to meet both the micro time-length measurement requirements with high resolution and the long time-length measurement requirements with low resolution, a conventional time measuring circuit can be provided with at least a pair of first and second time measuring modules. The first time measuring module is designed to implement micro time-length measurement with high resolution. In addition, the second time measuring module is designed to implement long time-length measurement with low resolution.
Specifically, the conventional time measuring circuit of another approach is configured to select any one of the first time measuring module and the second time measuring module depending on the intended use.
The different approach however may also cause a conventional time measuring circuit to increase in size.